The present invention relates to a diode which has a PNN.sup.+ structure and can operate at high speed.
Recently, there has been a strong trend to miniaturize various types of equipment and many of such power units included in the equipment have been changed from a conventional dropper type power source to a switching regulator. Further, the switching frequency of the regulator has been increased to 50 KHz or more, and a switching regulator having a frequency of 200 to 300 KHz will be put to practical use in the near future. In order to reduce switching loss, the switching regulator of this kind is required to include a high-speed diode which is short in reverse recovery time (trr) and small in forward voltage drop V.sub.F. Further, in order to reduce switching noise in the regulator, a high-speed diode is required in which a reverse recovery current rises gently to exhibit the so-called soft recovery characteristic.
Presently available high-speed diodes which are suitable for use in such switching regulators are classified roughly in two groups from the structural point of view. One of the groups is a Schottky barrier diode for a low voltage source such as a 5-volt power source. The Schottky barrier diode is advantageous in that the reverse recovery time (trr) is tens of nanoseconds or less since no minority carriers are stored, and in that the forward voltage drop V.sub.F can be reduced to a value of 0.5 to 0.6 V by appropriately selecting the height of the Schottky barrier. However, this diode has a drawback that the breakdown voltage V.sub.R is low, that is, it lies in a range from 40 to 50 V. The other group is a high-speed diode with a PNN.sup.+ (P.nu.N, P.pi.N or PIN) structure for relatively high voltage sources such as a 12-volt power source and a 24-volt power source. This diode has a reduced capacitance and a breakdown voltage of 100 to 200 V due to the existence of a high resistivity region between the cathode and anode, but is found to have the following drawbacks. The forward voltage drop V.sub.F is relatively high, that is, about 1 V, and the reverse recovery time (trr) is hundreds of nanoseconds since minority carriers are stored. Further, the present inventors have found that the reverse recovery current rises abruptly, and considerable noise is produced in the packaged diode because of this. It is to be noted that the soft reverse recovery is a new subject which is recognized by the present inventors as being important The present invention is directed to attaining such a soft reverse recovery.